The present invention relates to electromagnetic radiation shielding and more particularly to a shielding means comprising a non-conductive base material having thereon a combined electrolessly-deposited metal layer of copper over lain with a second layer.
It is often necessary to provide shielding material or enclosures for electronic and radio equipment which shields against electromagnetic radiation such as radio-frequency interference emitted from electronic and radio equipment therein. Regulations of the Federal Communication Commission require that certain categories of electronic and computer equipment be enclosed by radio-frequency interference shields. Electronic radio frequency radiation ranges from about 10 kiloHerz to 1,000 megHerz. Generally, shielding devices shield against a much wider frequency band of electromagnetic radiation.
It is known that electrically conducting materials will effectuate electromagnetic radiation shielding in general and radio-frequency interference shielding in particular. Accordingly it is common practice to provide a metal coating on enclosures for electronic equipment. Among the techniques known to apply a metal coating, are the use of electrically conductive paints, flame or arc spray metal techniques, cathode sputtering techniques, vacuum metallization, the application of metal foils, the electroless deposition of a metal layer, etc.
U.S. Pat. No. 4,169,171, issued Sep. 25, 1979 to Harold Narcus and assigned to the assignee of this application, discloses the use of an electroless metal coating over dielectric material such as ABS plastic or other materials, with the use of an electroless copper coating followed by a secondary electroless metal coating. The disclosure of U.S. Pat. No. 4,169,171 is directed to providing a finished product for a decorative use only, i.e., a bright metal outer surface.
Copper is known to be an economical and effective radio-frequency shielding metal, but has the disadvantage of darkening and surface oxidizing so that it is, per se, unsuitable for use as an effective metal shield. Pure copper shielding also oxidizes, and loses electrical conductivity at the mating surfaces of the enclosure. The oxidized copper surfaces become insulators and therefore act as slot antennas, leaking radio-frequency radiation, therefrom.
Generally, the effectiveness of electromagnetic radiation shielding i.e., radio-frequency interference shielding, is measured in decibels and generally, a 10 decibel attenuation corresponds to a signal reduction to about 90.9% of the unshielded signal; a 20 decibel attenuation is equivalent to a 99.0% reduction of the original signal and a 30 decibel attenuation is comparable to a 99.9% attenuation of the original signal, and so on.